Fronds Change Shape to Avoid Damage

Biological Strategy

Seaweed

Eleanor Banwell

Image: Claire Fackler / Public Domain ‑ No restrictions

Move in/on Solids

To obtain needed resources or escape predators, some living systems must move on solid substances, some must move within them, and others must do both. Solids vary in their form; they can be soft or porous like leaves, sand, skin, and snow, or hard like rock, ice, or tree bark. Movement can involve a whole living system, such as an ostrich running across the ground or an earthworm burrowing through the soil. It can also involve just part of a living system, such as a mosquito poking its mouthparts into skin. Solids vary in smoothness, stickiness, moisture content, density, etc, each of which presents different challenges. As a result, living systems have adaptations to meet one, and sometimes multiple, challenges. For example, some insects must be able to hold onto both rough and slippery leaf surfaces due to the diversity in their environment.

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Move in/on Liquids

Water is not only the most abundant liquid on earth, but it’s vital to life–so it’s no surprise that the majority of life has evolved to thrive on and under its surface. Moving efficiently in and on this dense and dynamic substance presents unique challenges and opportunities for living systems. As a result, they have evolved countless solutions to optimize drag, utilize surface tension, fine tune buoyancy, and take advantage of various types of currents and fluid dynamics. For example, sharks can slide through water by reducing drag due to their streamlined shape and specially shaped features on their skin.

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Attach Permanently

A living system can conserve energy by attaching permanently to a particular site because it can take advantage of resources that come its way, rather than expending energy to move to resources. A permanent attachment, intended to last the lifetime of the living system, creates special challenges. For example, physical mechanisms, such as the anchor that holds a marine algae to the ocean’s bottom, must be able to withstand forces that can pull it off its substrate. Chemical mechanisms, such as a barnacle’s glue, must avoid both physical and chemical breakdown, such as being dissolved by water.

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Manage Turbulence

A turbulent force occurs when air or water creates a chaotic or irregular motion. The source can be such things as wind, waves, and eddies caused by obstructions to air or water flow (such as that created by a rock in a stream). Because the force is irregular, it acts in unpredictable ways on multiple parts of a living system at any given time, decreasing the living system’s efficiency. Strategies used to manage turbulence include dampening the amount of turbulence, having flexibility to handle sudden changes, and making quick adjustments. An example is the mucus on aquatic organisms, such as barracuda sharks, that can reduce turbulent friction of seawater by 66%. In doing so, it decreases drag and increases the sharks’ swimming efficiency.

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Prevent Fracture/Rupture

High force impact or stress can cause materials that comprise living systems to separate into two or more pieces (called fracturing) or to break or burst suddenly (called rupturing). For example, a scallop prevents structural failure from fracture because its shell is comprised of two materials of varying stiffness. When a crack moves from the scallop’s stiff material to the less stiff one, the latter reduces the force at the tip of the crack, thereby stopping it from spreading farther.

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Plants

Phylum Plantae (“plants”): Angiosperms, gymnosperms, ferns, hornworts, liverworts, mosses, and green algae

Plants have evolved by using special structures within their cells to harness energy directly from sunlight. There are currently over 350,000 known species of plants which include angiosperms (flowering trees and plants), gymnosperms (conifers, Gingkos, and others), ferns, hornworts, liverworts, mosses, and green algae. While most get energy through the process of photosynthesis, some are partially carnivores, feeding on the bodies of insects, and others are plant parasites, feeding entirely off of other plants. Plants reproduce through fruits, seeds, spores, and even asexually. They evolved approximately 500 million years ago and can now be found on every continent worldwide.

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Fronds of seaweed change shape and orientation to allow them to move with instead of resisting strong water currents.

Introduction

Seaweeds, or macroalgae, often exist in or near the intertidal zone of oceans, where they are subject to very large forces from rapidly flowing tidal and storm currents. Unlike other occupants of intertidal zones, macroalgae are attached to the ocean floor and cannot move to take shelter. At the same time, they rely on energy from the sun to survive and so they need to maximize their surface area. This large frond area leaves seaweeds at high risk of damage.

Image: brewbooks / CC BY SA ‑ Creative Commons Attribution + ShareAlike

Seaweeds, or macroalgae, often exist in or near the intertidal zone of oceans, where they are subject to very large forces from rapidly flowing tidal and storm currents.

Image: Steve Lonhart / Public Domain ‑ No restrictions

Calcified seaweeds have joints that allow them to bend with the flow of water.

Image: Claire Fackler / Public Domain ‑ No restrictions

When exposed to strong currents, macroalgae bend closer to the ocean floor.

The Strategy

Intertidal macroalgae solve this problem by changing shape. When currents are mild, they project a large surface area into the water, gently wafting in the waves and collecting energy from the sun. However, when exposed to strong currents, macroalgae bend close to the floor and their fronds are swept into a narrow configuration that does not protrude into the current. Like a cyclist bending low over the handlebars, this more hydrodynamic shape reduces the interaction with the water, reducing stress on the organism and preventing damage and fracture.

The Potential

As with seaweed, too much rigidity in a material may lead to fracture or failure in the face of stress. Designing materials that can flex has many applications, including buildings, bridges, airplane wings, and even bioimplants. The ability to bend and change shape could one day result in smart materials that adapt to the whims of nature, changing properties to become more aerodynamic during high winds or becoming more flexible during earthquakes.

Body Changes Shape

Sea anemones

The central cavity of sea anemones is reinflated by water pumping in at low pressures thanks to ciliary pumps.

Biological Strategy

Change Increases Aerodynamic Performance

Common swift

Wings of gliding birds increase aerodynamic performance by continuously changing shape and size.

Last Updated July 23, 2019

References

Journal article

Mechanics without Muscle: Biomechanical Inspiration from the Plant World

Integrative and Comparative Biology | January 1, 2000 | Martone PT, Boller M, Burgert I, Dumais J, Edwards J, Mach K, Rowe N, Rueggeberg M, Seidel R, Speck T

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